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Bala Venkatesh*, Ingrid Hickman, Janelle Nisbet, Jeremy Cohen and John Prins
Corresponding author: Bala Venkatesh email@example.com
Critical Care 2009, 13:R105 doi:10.1186/cc7941
See related commentary by Owecki, http://ccforum.com/content/13/4/174
(2009-12-03 16:35) Boston University School of Medicine
We read with interest the report by Venkatesh et al investigating changes in adiponectin
during critical illness. Results show a significant relationship between adiponectin
concentration and cortisol levels in critically ill subjects (eg., an R2 of 0.32 with
p=0.01 at day 3). Upon further review of the data we observed that one extreme outlier
exists in the cortisol day 3 data set (level 2620 nmol/L, with all other values below
600 nmol/L). As the authors point out, this outlier skews the cortisol data to a
non-parametric distribution. Thus, in this case, the use of a simple linear regression
to analyze the relationship between adiponectin and cortisol violates the normality
assumptions needed for valid linear regression analyses. More appropriate analyses
of these data would include reporting the Spearman correlation coefficient (as mentioned
in Methods section) or log-transformation of the data for linear regression analysis.
Performing these analyses, we found the Spearman correlation coefficient for the relationship
between adiponectin and cortisol to be 0.51 with p=0.17; alternatively, linear regression
with log-transformed values led to an R2=0.06 with p=0.33. Results demonstrate that
when using non-parametric statistical methods, the relationship between adiponectin
and cortisol at day 3 is no longer statistically significant. Despite this, we agree
with the authors conclusion that “the relation between adiponectin and the inflammatory
response, organ dysfunction and outcome in critical illness” is a subject worthy
of future investigations. Sincerely, Allan Walkey, MD Ross Summer, MD
We declare no competing interests.
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